1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system such as a copier, a laser printer or a facsimile machine.
2. Description of the Related Art
As a developing unit used in an image forming apparatus of an electrophotographic system such as a copier, a facsimile machine or a printer, a developing unit in a toner concentration self control system is known in which toner concentration of two component developer including toner and magnetic particles used for developing an electrostatic latent image formed on an image carrying member is autonomously controlled, and toner concentration is kept to be a predetermined concentration.
In the developing unit in the toner concentration self control system, developer is charged before the developer is supplied to a developer carrying member, and the developer is not stirred in a longitudinal direction of the developer carrying member. As a result, in a case where a line which is parallel to a recording paper conveying direction (which is perpendicular to the longitudinal direction of the developer carrying member), a so-called longitudinal line, continues in an original image, a mixing rate (referred to as a toner rate, hereinafter) between toner and carrier of the developer decreases while the longitudinal line is being printed on recording paper. This is because, while the longitudinal line is being thus printed on the recording paper in the recording paper conveying direction, the developer on the developer carrying member does not move in the longitudinal direction of the developer carrying member to supplement consumed toner thus consumed to print the longitudinal line. As a result, image density may not be able to be kept at a backward portion of the recording paper in the recording paper conveying direction.
According to Japanese Laid-Open Patent Application No. 2005-173296, in an image forming apparatus having a two component developing unit in the toner concentration self control system, recording paper conveying intervals are adjusted according to a length of recording paper, and thus, a decrease in image density caused by toner rate reduction occurring when long recording paper is used is avoided.
However, when the recording paper conveying intervals are increased as an image is formed on long recording paper, a long time may be required for a printing process during which the image is formed on the recording paper and the recording paper is then ejected.